In recent years, the demand for a tape-shaped magnetic recording medium comprising a magnetic recording layer (hereinafter referred to as "magnetic layer") for use e.g., in audio equipments, video equipments and computers and a disk-shaped magnetic recording medium comprising a magnetic layer such as a floppy disk has drastically grown regardless of whether they are for general purpose or business purpose. With this drastic increase in the demand for magnetic recording media, there has been a great increase in the amount of defectives and waste articles from the production process and used articles.
This type of a magnetic recording medium generally comprises a magnetic layer having a ferromagnetic powder dispersed in a binder on a film support such as polyester. Therefore, the recovery and reuse of such a film support such as polyester is very important in view of environmental protection.
Such demand for environmental protection is not limited to magnetic tapes. Recovery has been also studied for other materials. For example, in the field of photographic light-sensitive materials, silver and film base are recovered. JP-A-52-138131 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a film base recovery process which comprises allowing ultrasonic wave to act on recovered waste film articles in a peeling bath for peeling off a light-sensitive layer and/or subbing layer from a support with an alkaline solution so that the peeling can be carried out with a low concentration alkaline solution in a short period of time. Specifically a 26 KHz ultrasonic wave in used to carry out the peeling by dipping in a 2% NaOH aqueous solution at a temperature of 70.degree. C. in 3 minutes.
JP-A-62-160451 proposes a process which comprises dipping a photographic film in an enzyme solution having an activating effect in an alkaline range so that gelatin is decomposed to separate silver from a film base.
JP-A-52-97738 proposes a process for removal of gelatin emulsion film from silver salt photographic films using base films composed of polyester and cellulose derivative by treating with an aqueous solution of an enzyme and the removal of a subbing layer from the mixture with an alkaline aqueous solution.
However, the emulsion layer in these photographic films has little or no void as compared to general magnetic layers and thus greatly differ from magnetic layers having a void of 40% or less at maximum. This produces a great disadvantage in peeling. In other words, the alkaline aqueous solution first dissolves the emulsion layer and then dissolves the subbing layer. This means that the alkaline aqueous solution cannot permeate through the emulsion layer to dissolve the subbing layer. Therefore, it takes much time for the alkaline solution to dissolve and peel off the emulsion layer and the subbing layer.
Such an approach with an alkaline solution to peel off a magnetic layer has been applied to commonly available magnetic tapes. For example, JP-A-53-70404 proposes a magnetic tape recovery and processing which comprises dipping a magnetic tape in a caustic alkaline solution having a concentration of 0.5% or more at an elevated temperature of 70.degree. C. to separate a magnetic material from a plastic film. There is disclosed that peeling cannot be effected with a weak alkali such as sodium carbonate or a low concentration alkaline solution. In its examples, peeling is effected under conditions of a 5 wt % NaOH solution at a temperature of 95.degree. C. in 3 minutes. This shows that the magnetic tape used in the examples exhibits a low peel rate as 3 minutes with a high concentration alkaline solution at an elevated temperature.
JP-A-53-112979 proposes a base film recovery process which comprises peeling with an aqueous solution containing 0.1 to 40% by weight of a basic substance at a temperature of 5.degree. to 150.degree. C. In its examples, peeling was effected under conditions of a 5 wt % NaOH solution at a temperature of 88.degree. C. in 100 minutes or with a 4 wt % NaOH solution at a temperature of 100.degree. C. in 10 minutes. This shows that the magnetic tape used in the examples exhibits an extremely low peel rate.
Further, JP-A-54-66985 proposes a magnetic tape recovery process which comprises dipping a magnetic tape in a strong alkaline solution at a temperature of 25.degree. C. to peel a magnetic layer. An aqueous solution of NaOH having a concentration of 5% by weight or more is used as the alkaline solution. In its examples, among five kinds of magnetic tapes tested, one treated under conditions of 90 minutes at a temperature of 60.degree. C. with a 5 wt % NaOH solution has the highest peel rate.
JP-A-60-223835 proposes a process which comprises addition of a basic substance as well as a surface active agent during peeling. In its examples, the peeling is effected under conditions of a 5 wt % NaOH solution at a temperature of 80.degree. C. in 10 minutes. Similarly, the peel rate thus attained is low.
As processes for the recovery of a base film, there are proposed in JP-A-59-52433 and JP-A-59-91028 starting material recovery processes which comprise cutting a magnetic tape, dipping the pieces in an alkali to swell them, stirring the material at a high speed so that the magnetic layer is separated from the base film, washing the materials, drying the materials, and then magnetically separating the materials. In their examples, an aqueous solution of NaOH having a concentration of 5 to 20% by weight is used. There is disclosed that the magnetic tape used exhibits a peel rate of 20 minutes with a 5 wt % NaOH solution at a temperature of 90.degree. C.
It can thus be seen that the prior art methods for peeling magnetic layer from waste magnetic tapes, an aqueous solution of alkali provide an extremely low peel rate, and magnetic tapes themselves are not provided with special design for peeling.
While the detailed description of the above cited prior arts state that the material can be treated with an aqueous solution having a wide range of concentrations, it can be seen that magnetic tapes disclosed therein do not exhibit such a high peel rate.
An attempt has been made to use special solvent to effect peeling. For example, JP-A-53-92879 and JP-A-53-94381 propose a base film recovery process which comprises processing a magnetic tape with a glycol solution containing 0.01 to 5% by weight of a basic substance (e.g., caustic soda, sodium carbonate, pyridine) at a temperature of 50.degree. to 200.degree. C. However, in their examples, the processing is effected with ethylene glycol at an elevated temperature of 100.degree. C. for 60 minutes or 145.degree. C. for 20 minutes. Under these processing conditions, a polyethylene terephthalate (PET) base film is damaged and the processing cost rises. In other words, ethylene glycol is a starting material for PET and has an extremely high solubility to PET.
JP-A-62-167601 proposes a base film recovery process which comprises dipping the material in a mixture of an organic amine and dimethyl formamide (DMF) to decompose the binder. However, the mixture of an organic amine and DMF cannot be practically used in view of the cost and installment. DMF is a solvent having an extremely high ability to produce a homogeneous system and can dissolve and penetrate PET itself, making it impossible to obtain recovered PET with good qualities.
As a process for the recovery of a base film from a thin metal film type magnetic recording medium, there is proposed in JP-B-1-32295 (the term "JP-B" as used herein means an "examined Japanese patent publication") a process which comprises processing the material with an alkaline solution. That is, a magnetic recording medium recovery and processing is proposed which comprise separation of a magnetic material and a base film which constitute a thin ferromagnetic metal film. The peel rate thus attained is 30 minutes with a 0.5N (2 wt %) NaOH aqueous solution at a temperature of 90.degree. C. Thus, thin metal film type magnetic recording media free of water-soluble subbing layer exhibit an extremely low peel rate.
As mentioned above, various efforts have been made to recover a base film from commercially available magnetic recording media. However, since ordinary magnetic media comprise a subbing layer for only purpose of firmly adhering the nonmagnetic plastic support to the magnetic layer, the magnetic layer cannot be actually peeled from the support unless an extremely strong alkaline aqueous solution or special organic solvent is used. When such a strong alkaline aqueous solution is used, the base film itself is hydrolyzed and the base film thus recovered does not exhibit sufficient qualities for reuse.
In the mechanism of peeling of the prior art methods, the magnetic layer, which comprises a binder cured with a polyisocyanate and insusceptible to hydrolyzation, is little dissolved in an alkaline aqueous solution. The alkaline aqueous solution penetrates the material through voids in the magnetic layer to hydrolyze a polymer such as polyester which constitutes the subbing layer to initiate peeling. At the same time, the polyester base (such as PET) itself begins to be hydrolyzed. Thus, the prior art processing with alkali is disadvantageous in that peeling proceeds with hydrolyzation rather than dissolution, providing a low peel rate. Further, the polyester base film thus recovered shows deterioration in qualities due to hydrolyzation.
On the other hand, various binders have been developed merely to improve the adhesion strength but regardless of peeling properties. For example, in JP-A-62-185227, JP-A-62-195064, JP-A-63-108530, JP-A-63-146213 and JP-A-63-108079, a coating composition for subbing layer using a polyester, in which 5 to 20 mol % of the dicarboxylic acid component is an ester-forming dicarboxylic acid having a sulfonic acid alkaline metalsalt group as a substituent, in combination with carbon black. The carbon black is used to improve electric conductivity, and a water soluble polymer, particularly a sulfonic acid metal salt substituted polyester resin, is used to improve the dispersibility of the carbon black and the adhesion strength. However, because the metal salt of sulfonic acid is completely substituted by alkaline metal salts, the solubility of the polymer in water and that in an alkaline aqueous solution are not changed. Therefore, if peelability is increased, the solubility in water of the polymer is also increased, resulting in that the tape thus-obtained is readily deteriorated by humidity.
JP-A-56-87233, JP-A-61-22423 and JP-A-58-217546 utilize a copolymerized polyester resin containing metal salts of sulfonic acid in a subbing layer. However, both the humidity resistance and the peelability cannot be improved at the same time because sulfonic acid is completely in the form of metal salt groups.
A subbing layer using water soluble polymers is disclosed, for example, in JP-A-61-13435 and JP-A-62-66422. However, because resins having high water solubility are used, both the humidity resistance and the peelability cannot be improved at the same time.
A resin containing carboxyl groups has been used in a subbing layer as described, for example, in JP-B-47-15623, JP-B-60-31011, JP-A-55-55435 and JP-A-2-278511. However, because an isocyanate compound is used in combination, the resin contained in the subbing layer becomes macromolecules that is highly cross-linked. The carboxyl group itself reacts with the isocyanate compound. Therefore, the subbing layer has no solubility in an alkaline aqueous solution.
JP-A-63-182381 discloses a subbing layer coating composition that is an aqueous composition containing a resin having at least one kind of a polar group and at least one unsaturated double bond per one molecule. However, this composition is finally cured by irradiation, and therefore a water-insoluble cross-linked three-dimensional macromolecule is formed, resulting in a subbing layer that is not soluble in an alkaline aqueous solution.
As mentioned above, there have been proposed various compounds as binders for subbing layer. However, since these subbing layers are all designed for only purpose of improving the adhesion between the nonmagnetic plastic support and the magnetic layer and hence improving durability, they are not quite intended for peeling of the magnetic layer the nonmagnetic plastic support by processing with an alkali. When attempted to peel the components, it is quite impossible, or even if possible, peeling cannot be effected unless processing is effected with a strong alkali such as high concentration aqueous solution of NaOH for an extended period of time. As a result of such a prolonged processing with a strong alkali, hydrolyzed PET with deteriorated qualities can only be recovered. Further, polymers having high water-solubility are poor in humidity resistance.
Other examples of subbing layers containing water-soluble groups include easily-adherable subbing layer as described below. For example, JP-A-62-204939 proposes an easily-adherable polyester film comprising a primer layer with an initial Young's modulus of 40 to 20,000 kg/cm.sup.2 made of an aqueous polyurethane resin and an aqueous acrylic resin. Further, JP-A-62-204940 proposes an easily-adherable polyester film comprising a primer layer made of an aqueous polyurethane resin and an aqueous acrylic resin. Moreover, JP-A-61-175925 proposes a magnetic recording medium comprising a compound containing either or both of sulfonic acid groups and sulfonate groups. These easily-adherable subbing layers are all obtained by stretching the above mentioned easily-adherable layer. Therefore, the easily-adherable layer lies partially on the nonmagnetic plastic support. Thus, even if such an easily-adherable layer is composed of water-soluble group or resin, peeling cannot be easily effected even with an alkaline aqueous solution because the easily-adherable layer lies partially on the support.
As mentioned above, the prior art subbing layer in magnetic recording media has been intended for improvement in the adhesion between the magnetic layer and/or back layer and the nonmagnetic plastic support but with no consideration of peelability of these layers. In other words, no attempts have been made to improve magnetic recording media themselves, particularly subbing layer, for the positive purpose of environmental protection and base film recovery.